In radio receivers such as radios, televisions, or mobile phones, deterioration of reception quality due to multipath noise becomes a problem. The multipath noise is caused by receiving the same radio waves (delayed waves) passing through a plurality of paths because a radio wave transmitted from a broadcasting station or base station is reflected or diffracted by obstacles such as buildings or ground.
In particular, as for an onboard FM radio receiver, since it is mounted on a vehicle and moves, its receiving conditions involving multipath occurrence fluctuate moment by moment. Accordingly, suppressing the multipath noise by carrying out appropriate control in accordance with the receiving conditions is an important design matter.
As to a method of suppressing the multipath noise by carrying out appropriate control in accordance with the receiving conditions, many applications have been submitted previously. For example, although an occurrence of the multipath can bring about rasping noise (distortion) because of an increase of high frequency components of an audio signal, a method is known for attenuating the high frequency components to counteract the noise of the high frequency components. In addition, an occurrence of the multipath can cause a harmful effect called stereo distortion that hampers correct stereo separation because of a distortion effect on a stereo pilot signal. Against the stereo distortion, a method is known which produces a monaural signal (see Patent Document 1 for both methods).    Patent Document 1: Japanese Patent Laid-Open No. 2003-69436 (Paragraphs [0006]-[0007]).
FIG. 13 shows an internal configuration of a typical conventional FM radio receiver. In FIG. 13, FM waves received with an antenna 1 are amplified with a radio frequency amplifier (RF) 2, pass through frequency conversion with a frequency converter (MIX) 3, and are deprived of undesired components such as adjacent channel waves through an intermediate frequency filter (BPF) 5. The FM wave passing through the intermediate frequency filter 5 is subjected to the amplitude limitation with a limiting intermediate frequency amplifier 6 (IF-AGC) and to the removal of high frequency components of the intermediate frequency with a pre-filter (LPF) 7, and is converted to a digital signal through an analog-digital converter (A/D) 8. Then, the FM wave converted to the digital signal is demodulated with an FM demodulator 9, is converted to an analog audio signal with a digital-analog converter (D/A) 10, and is supplied to a speaker 12 via an audio amplifier 11.
In the foregoing configuration, although the multipath occurrence brings about the rasping noise because of an increase of the high frequency components of the audio signal (distortion), a frequency characteristic setting section (HCC) 23 attenuates the high frequency components to counteract the noise of the high frequency components. In addition, although the harmful effect called stereo distortion occurs which hampers the correct stereo separation because of the effect of the distortion on the stereo pilot signal, a separation setting section (SRC) 22 carries out appropriate control in accordance with the receiving conditions by producing a monaural signal against the stereo distortion, thereby suppressing the multipath noise.
Thus, according to the output of a bandpass filter (BPF) 16, the separation setting section 22 and the frequency characteristic setting section 23 are controlled in accordance with the noise occurrence states. Besides, to suppress the occurrence of the multipath noise itself, the operation limitations are imposed on the amplitude correction in the FM demodulator 9.
As control parameters for the operation limitations on the amplitude correction, using the receiving conditions based on the received field level and modulation signal level, a receiving condition deciding section 20 decides the receiving conditions from the output of a received field level detecting section 18 and the output of a modulation signal level detecting section 19, and carries out the control of the operation limiting level in accordance with the receiving conditions.
As described above, according to the conventional FM radio receiver, it suppresses the multipath noise by imposing the operation limitations on the amplitude correction in the FM demodulator. In addition, as the control parameters of the operation limitations on the amplitude correction, it employs the receiving conditions based on at least one of the received field level and modulation signal level or both of them.
However, the control parameters do not take into account the multipath occurrence state. Accordingly, a problem arises, for example, in that the multipath distortion can occur in strong multipath occurrence state because of the insufficient operation limiting level of the amplitude correction, but that the audio distortion can occur in weak multipath occurrence state because of the excessive operation limiting level of the amplitude correction.
The present invention is implemented to solve the foregoing problems. Therefore it is an object of the present invention to provide a radio receiver capable of further improving the reception quality by suppressing the multipath noise by carrying out appropriate control in accordance with the receiving conditions taking the multipath occurrence state into account.